This invention relates to rotary drum filters and more particularly to vacuum filters having an externally secured valve generally located near the axial center of the drum filter.
At various times in processing of papermaking pulp, it is necessary to thicken or otherwise increase consistency of the pulp slurry. This is commonly accomplished on a drum filter having a perforated deck overlaying a series of axially oriented liquid channels which, as the drum rotates, sequentially conduct liquid towards a stationary valve which regulates the timing for discharging liquid from the channels and its eventual removal from the filter unit. In the case of a center valve filter such as considered here, the stationary center valve is held in a cooperating relationship with the open ends of the fluid channels and contained within a circumferential passageway about the drum bounded on its outer circumference by cover plates attached to the drum. As the drum rotates, the valve allows liquid to sequentially drain from the channels as they pass from sealing relationship with the ends of the valve and allow liquid to flow into the center of the drum. A sealed relationship between the stationary valve and channel ends is required to arrest drainage of the channels and prevent atmospheric air from leaking into the drum center. Additionally, the edges of the circumferential cover plates and the valve holding shoe must be sealed to prevent atmospheric air from entering the vacuous interior of the drum.
In center valve filters currently in use, sealing the slot between the stationary valve circumferential cover plates may be provided by a circumferential endless flat belt-like seal which covers the slot and rides over a valve holding shoe or alternatively by abutting flexible seal members, mounted on opposing circumferential cover plates, arranged to touch and thereby seal the slot and intended to maintain sealing contact as they move around the outer surfaces of the stationary valve holding pin. In either case, the required vacuum necessary to attain pulp thickening capacity and consistency has been subject to rapid degradation and loss due to the seal member losing its flexibility and elasticity as well as being subject to failure of the jointing of the ends of the sealing members as a result of temperature and chemical attack. In addition, wear of the sealing members and, in the case of the flat belt-like seal, uncovering of the slot between the cover plates due to wandering and/or curling of the sealing belts inhibits and/or causes drainage failure due to the ingress of air reducing the vacuum. Also as the material of the seal degrades, fragments of the seal material sluff off into the pulp slurry and cause paper defects.
The pulp contamination and/or the reduced filtering capacity require shutdown of the filter for seal replacement. Since standby equipment is never available, such shutdown results in a significant financial loss arising from lost production as well as a significant expense for the required disassembly and repair.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.